Thermocouples used for high temperature measurements are generally made of noble metals such as platinum and alloys of platinum. Most common are: (1) Type R thermocouples having a positive lead material of platinum and 13% rhodium and a negative lead of platinum, and (2) Type S thermocouples having a positive lead material of platinum and 10% rhodium and a negative lead of platinum.
A disadvantage of noble metal thermocouples is their high cost, the cost being proportional to the length of the platinum and platinum/rhodium thermocouple wires.
Inn an industrial environment, the instrumentation at which the reference junction is located is typically distant from the thermocouple. In particular, where the temperature of molten metals is being measured, the instrumentation used to measure the emf output of the thermocouple is typically at a great distance, e.g. 100 feet, from the measurement junction of the thermocouple. To extend the noble metal thermocouple 100 feet or more to the measurement junction would be prohibitively expensive. Further, where for instance, the thermocouple is used for only one or a few measurements, such as is typically the case in the temperature measurement of molten metals, the use of a noble metal thermocouple which would extend for 100 feet or more becomes even more prohibitive.
Temperature measurement systems which use a noble metal thermocouple for measuring the temperature of molten metals generally minimize the length of the noble metal thermocouple wires by attaching, in a connecting device frequently referred to as a sensor or probe, connecting wire made of less expensive metals/metal alloys, to connect the thermocouple to the distant measuring instrument. Such connecting wire is commonly referred to as extension wire.
Extension wire may be used to connect a thermocouple to a distant measuring instrument, while retaining acceptable measurement accuracy for most applications by: (1) selecting the thermoelectric properties of the extension wire to be substantially the same as those of the noble metal thermocouple to which it is attached over the temperature range at which the extension wire operates, and (2) maintaining the junction of the positive thermocouple wire and the positive extension wire to be at the same temperature as the junction of the negative thermocouple wire and the negative extension wire. Under these conditions, the voltage measured at the reference junction is ideally a function of only the temperature difference between the measurement junction and the reference junction irrespective of the temperature of junctions formed at the connection of the thermocouple wires and the extension wires. Conventionally, the extension wire for connecting to the positive thermocouple lead of a noble metal thermocouple is made of pure copper, and the extension wire connecting to the negative lead is made of a copper-nickel alloy. The selection of a particular type of material for matching the thermoelectric properties of one type of extension wire with that of a noble metal thermocouple for minimizing errors due to the mismatch of the thermoelectric properties between the thermocouple and the extension wires is described in U.S. Pat. Nos. 3,926,681 and 4,002,500. The measurement error due to a difference in the temperatures at the junctions of a thermocouple and the extension wires due to mismatching of the thermoelectric properties of the extension wire to the thermocouple element is discussed in the “Manual on the Use of Thermocouples in Temperature Measurement”, ASTM Pub. 470B, 1981, pages 27-35.
As discussed above, a temperature difference arising between the positive and negative junctions of the thermocouple and the extension wire (hereafter referred to as intermediate junctions) results in an error of the measurement of the thermocouple emf. Not discussed in the prior art is that this temperature difference between the intermediate junctions may arise from a difference in the heat flowing from the positive intermediate junction to the cooler corresponding reference junction and the heat flowing from the negative intermediate junction to the cooler corresponding reference junction. The difference in heat flows results in an apparent cooling of one of the junctions under conditions where both junctions are receiving an equal heat input. This temperature difference will increase in proportion to the time that the thermocouple, more specifically the intermediate junctions, are exposed to a circumstance of progressive heat gain as a result of exposure to the high temperature environment.
Increasing the accuracy of noble metal temperature measurements, and in particular, increasing the accuracy of the temperature measurement of molten metals would be an economic advantage to industry. Consequently, reducing the differential heat flow from the junctions of the thermocouple and the extension wires through the thermocouple extension wires would be desirable.